1. Field of the Invention
The present invention relates to a high speed transimpedance amplifier, and particularly to a high speed transimpedance amplifier that can utilizes a resistive feedback characteristic to increase sensitivity of the high speed transimpedance amplifier, or decrease power consumption of the high speed transimpedance amplifier.
2. Description of the Prior Art
A transimpedance amplifier is an important component in a receiver of an optical communication system, where noise and bandwidth performance of the transimpedance amplifier can determine how far and how fast the optical communication system can reach. Please refer to FIG. 1. FIG. 1 is a diagram illustrating a transimpedance amplifier 100 according to the prior art. As shown in FIG. 1, the transimpedance amplifier 100 includes three identical inverters 102, 104, and 106, three diode-connected transistors LM1, LM2, and LM3, and a feedback resistor RF, where the transistors LM1, LM2, and LM3 are used for increasing bandwidth of the transimpedance amplifier 100, and a gain A of the transimpedance amplifier 100 can be determined by equation (1):
                    A        =                                            g                              mLM                ⁢                                                                  ⁢                1                                                                    g                                  mPM                  ⁢                                                                          ⁢                  1                                            +                              g                                  mNM                  ⁢                                                                          ⁢                  1                                                              ×                                    g                              mLM                ⁢                                                                  ⁢                2                                                                    g                                  mPM                  ⁢                                                                          ⁢                  2                                            +                              g                                  mNM                  ⁢                                                                          ⁢                  2                                                              ×                                    g                              mLM                ⁢                                                                  ⁢                3                                                                    g                                  mPM                  ⁢                                                                          ⁢                  3                                            +                              g                                  mNM                  ⁢                                                                          ⁢                  3                                                                                        (        1        )            
As shown in equation (1), gmLMi, gmPMi, and gmNMi represent transconductances of transistors LMi, PMi, and NMi, respectively, where 1≦i≦3, and i is a positive integer. Therefore, the transimpedance amplifier 100 utilizes gains of the inverters 102, 104, and 106 to obtain a high gain. As shown in FIG. 1, input impedance RIN and a transimpedance gain TZ of the transimpedance amplifier 100 (from an input current Iin to an output voltage VOUT) can be determined by equation (2) and equation (3), respectively:
                    RIN        =                  RF          A                                    (        2        )                                TZ        =        RF                            (        3        )            
If an input capacitor CIN includes a parasitic capacitor of a photodiode 108 and other parasitic capacitors of an input terminal of the transimpedance amplifier 100, 3-dB bandwidth f3dB of the transimpedance amplifier 100 can be determined by equation (4):
                              f                      3            ⁢            dB                          =                              1                          2              ⁢              π              ×              RIN              ×              CIN                                =                      A                          2              ⁢              π              ×              RF              ×              CIN                                                          (        4        )            
In addition, input noise Inoise of the transimpedance amplifier 100 can be determined by equation (5):
                              I          noise                =                                            4              ⁢              kT              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢              f                        RF                                              (        5        )            
As shown in equation (5), T is an absolute temperature, k is the Boltzmann constant, and Δf is bandwidth for a predetermined data transmission rate. In addition, it is noted that the input noise Inoise of the transimpedance amplifier 100 is an indicator of sensitivity of the transimpedance amplifier 100. Therefore, the input noise Inoise is smaller, the sensitivity of the transimpedance amplifier 100 is better.
Equation (4) and equation (5) can be regarded as a design guide of the transimpedance amplifier 100. Therefore, the gain A and the feedback resistor RF of the transimpedance amplifier 100 can determine the 3-dB bandwidth f3dB and the input noise Inoise of the transimpedance amplifier 100 according to equation (4) and equation (5). However, because the gain A of the transimpedance amplifier 100 is limited to the three inverters 102, 104, and 106 (that is, the gain A of the transimpedance amplifier 100 generated by the three inverters 102, 104, and 106 is still not enough), the transimpedance amplifier 100 is not a good choice for a designer of the optical communication system.